Hmm, a few thoughts here and there:
--You probably know that a basic alarm system is fairly cheap, right? From a financial point of view, this project is hardly worth the time.........but if you're doing the project just for your own enjoyment, that's cool. I like projects too.
--The horn wire, inside the car, is a low-current ground trigger---it'll be one of the black wires at the steering column on a GM car. So, you shouldn't need that second relay at all.........just have that first relay, so that the ground from the toggle switch passes through 87 30 ONLY when the door is open..... that'll activate the horn.
--Two examples of pulse-timer relays that should suit your need, to disable the horn-honking after a period of time:
DEI's 528T:
http://www.caraudio-caralarms.com/528T.html
PAC TR-7. I've never used one, but it seems like a really cool part:
http://www.soundoffaudio.com/item.cfm/id/2084
--Also, more than just limiting the duration of the horn..... you also probably want the horn to pulse on and off, rather than a continuous blast......horns are not designed for continuous operation; it
might burn out.
For that, I would suggest an electronic (not thermal) turn-signal flasher from an auto parts store. I think they are designed for a positive, high-current trigger......so you may HAVE to get the positive horn wire under the hood, rather than the negative one at the steering column.
--You do know that whatever switch you choose to mount outside the car, would have to be waterproof, right? If you don't object to drilling a hole in your fender, check out the key-operated switches, like the used in the '70s.
--As far as your other switch, and your test light....there is an easier way.
Purchase a 12-volt, flashing LED, and wire it up to receive power & ground whenever your system is "armed."
You can buy such an LED as a fake car alarm from eBay or J.C. Whitney, or just the raw LED from Radio Shack or Parts Express. They are not hard to find.
Not only will you save the trouble of an extra switch, you'll also have an added theft-deterrent when potential thieves see the flashing light.
--Not sure if you know this, but some remote starters have a wire that puts out a ground ALL THE TIME during the time that the doors are locked. This would be to hook up an optional starter kill setup.
Take your circuit tester, and try it out on each of the unused wires from your remote starter, while locking/unlocking the doors with the remote...........if your unit has such an output, it would save you all the trouble of that hidden switch, and be easier to use as well.
--The way you have your relays set up in that diagram looks fine. But just a reminder in case you didn't know it------you mustn't set up a relay that stays on all the time when the car is off; it will eventually drain the battery.
--What are you using to make your wire diagrams? I want to be able to do the same.